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From Cams To Carbs

Discussion in 'Technical Q&A' started by pernicev, Jul 7, 2005.

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  1. pernicev

    pernicev Karting

    Jun 22, 2005
    134
    #1 pernicev, Jul 7, 2005
    In the continuing saga of my rear bank inlet cam with 2 marks, I've come to an impasse. The marks were close together and I chose the one that best corresponded with the mark on the flywheel, but I am still not so sure… I have everything back together and I set the ignition timing and synched the carbs today. All seems well when the engine is warmed up, there is very little popping from the carbs. The problem is that my idle seems to be hunting and when I rev the engine, it decelerates slowly. Am I wrong to still think that the cam timing may be out? I should also mention that I just cleaned the carbs by soaking in carb dip and rebuilt them.
     
  2. marks308GTB

    marks308GTB Karting

    Jun 6, 2004
    114
    Los Altos Hills, Ca.
    Full Name:
    Mark S.
    #2 marks308GTB, Jul 7, 2005
    Had the same problem on mine. Turned out the rear bank cam belt was off one tooth.

    Mark
     
  3. pernicev

    pernicev Karting

    Jun 22, 2005
    134
    #3 pernicev, Jul 7, 2005
    Thanks, that would suport the cam timing theory. Any other opinions?
     
  4. Mike328

    Mike328 F1 Rookie

    Oct 19, 2002
    2,655
    Boulder, CO
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    Mike
    #4 Mike328, Jul 7, 2005
    If when you rev the engine, with the car in neutral, the RPMs are slow to drop, this could be indicative of an air leak anywhere in the system, perhaps where the carbs meet the intake runners. Another option is, the carbs ain't synced right. Have you synced these carbs before, or is this your first time?
     
  5. pernicev

    pernicev Karting

    Jun 22, 2005
    134
    #5 pernicev, Jul 7, 2005
    Not the first time I've synched 308 carbs by any means, but the first time on this one. A little history: I bought this low mileage car in a non-running condition as it was sitting for over a year. I cleaned out the gas tanks, pulled and cleaned the carbs. The car ran like a champ after that and the carbs weren't even synched yet. The belts were old, so I immediately did a major, including two exhaust valves that accidentally broke (probably when I put on my cams). With everything back together , I've fiddled with setting different ignition timing, but it makes no difference with the carb problem. Other than what I've mentioned, this 308 is the most mechanically solid that I've had the pleasure of working on.
     
  6. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
    Chicago
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    Philip
    #6 pma1010, Jul 7, 2005
    Vince, I am not help outside of stating the obvious. You have to get comfortable with the cam timing -- sounds like time for a degree wheel.
    Philip
     
  7. Artvonne

    Artvonne F1 Veteran

    Oct 29, 2004
    5,379
    NWA
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    Paul
    #7 Artvonne, Jul 7, 2005
    I second the degree wheel in finding your marks. Degree wheeling the engine is a requirement in the WSM, but it also explains the marks will suffice "in a pinch". But as you seem to have two marks you will need a wheel to determine which is the correct mark. Will the real timing mark please stand up?
     
  8. Matt Morgan, "Kermit"

    Matt Morgan, "Kermit" Formula Junior

    Nov 12, 2003
    405
    Ferndale, WA
    #8 Matt Morgan, "Kermit", Jul 8, 2005
    FWIW: Paul and Phil hit it right on the nose, it's time to get out the wheel and dial indicators, providing you want it done right. Don't be suprised if right off the bat you discover the timing marker is of when you find true TDC. File one side to fix that. It all takes a bit of time, but it is definately worth it.
    One thing I have noticed on several 2 valve motors, is there is a mark on the pulley flange, just in line with the right cam mark. Look, and maybe you are lucky!
    HTH,
    Kermit
    BTW: The carb cams can be moved around a bit and will give you a bit more top end power, withhout sacrificing lower That is the beauty of the 308 cam design, and why we "dial em".
     
  9. pernicev

    pernicev Karting

    Jun 22, 2005
    134
    #9 pernicev, Jul 8, 2005
    I agree with all of the above, but what I was actually asking is if this decelleration problem was most likely a mis-timed cam and not a carb issue. I had run into this problem before, but that was six years ago and I can't seem to remember if it was the cams or not. Must be all those fumes.
     
  10. pma1010

    pma1010 F1 Rookie

    Jul 21, 2002
    2,559
    Chicago
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    Philip
    #10 pma1010, Jul 8, 2005
    Vince:
    If you knew the cam timing was ok, I'd opine that if it hangs high/returns slowly to idle it is rich (too rich) and therefore wants to speed up. Take idle jets down a notch.
    Philip
     
  11. Ken

    Ken F1 World Champ

    Oct 19, 2001
    16,078
    Arlington Heights IL
    Full Name:
    Kenneth
    #11 Ken, Jul 8, 2005
    Hunting at idle can be a vacuum leak around the carbs...

    My Lotus flywheel has two marks as well. I wonder why they DO that???

    Ken
     
  12. Matt Morgan, "Kermit"

    Matt Morgan, "Kermit" Formula Junior

    Nov 12, 2003
    405
    Ferndale, WA
    #12 Matt Morgan, "Kermit", Jul 8, 2005
    There is one "unless" I might ad to that. If by chance you are running an Electromotive HPX, they are a "Wasted Spark" type ignition, much like the old Harleys. That means they fire one on time, and at the same time a second gets a spark, but it is on the exhaust stroke, and if there is any unburned fuel. Bango! Too rich on the idle circuit, as with the motor pulling hard against a closed throttle plate on coast down will really make them "talk" to ya.
    The motor being used as a compression brake in a way, causes a high vacuum in the intake, thus not much air, but it sucks hard on the gas.
    HTH,
    Kermit
     
  13. Steve King

    Steve King F1 Rookie

    Feb 15, 2001
    4,367
    NY
    #13 Steve King, Jul 8, 2005
    When was the last time you check the advance mech. in the dizzys. Might be gummed up or oil leaking in and causing the problem. With a timing light on check the advance from 1000rpm up to 5000rpm and watch as the motor decelerates.
     

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